Our recent linkage study in Pima Indians provided evidence for a gene determining insulin action located near two markers on chromosome 4q, the intestinal fatty acid binding protein gene (FABP2), and the annexin V gene (ANX5). Extensive typing of numerous polymorphic markers on 4q narrowed this putative gene to a chromosomal segment about 5 centiMorgan long (an estimated 5x10/6 base pairs). Because FABP2 and ANX5 are the only well- positioned functional genes in this region, two strategies are being pursued to search for the putative gene. First, we are exploring the FABP2 gene itself as a potential candidate because of the known role of fatty acids in insulin action. Analysis of FABP2 coding sequences by the single-strand conformation polymorphism technique (SSCP) detected a single base substitution (G --> A) in the second exon resulting in an Ala to Thr substitution at position 54 of the protein. The structural and functional significance of this substitution is currently under investigation. Second, we have initiated a search for new genes in the FABP2/ANX5 region by using the yeast artificial chromosome (YAC) genomic cloning system. Molecular analysis of a YAC clone carrying the ANX5 gene led to the discovery that the cyclin A gene (CCNA), previously assigned to 4q by in situ hybridization, is located within 150 kilobases from ANX5.